(a) Field of the Invention
The invention relates to ventilation systems for turbines.
More specifically, the invention relates to a ventilation system capable of adapting to suit different air extraction flow rates.
One particularly beneficial application of the invention relates to a ventilation system for a gas turbine.
(B) Description of the Related Art
Gas turbines, particularly industrial turbines developing powers of between 40 and 130 megawatts, generally use ventilation systems in order to avoid thermal deformation in the spool or stator of the turbine.
A ventilation system generally comprises at least one motor-driven fan and air inlet and extraction ducts connected to the turbine compartment. It also comprises a device for attenuating nuisance noise acting on the ventilation air inlet and extraction ducts.
As environmental standards and European directives are becoming increasingly tight, ventilation systems operating in an “explosive atmosphere” are subject to very strict regulations, dictating the design of such ventilation systems.
What is meant by an “explosive atmosphere” is an atmosphere that could become explosive as a result of local conditions (the presence of air, fuel, and a source of heat or a spark). This is a mixture of air with flammable substances in the form of gas, vapour or dust, in which, following ignition, combustion spreads throughout the mixture of unburnt gases.
In order to comply with all these standards and directives, the ventilation system needs either to prevent an explosive atmosphere from forming or, where this is not possible, prevent an explosive atmosphere from igniting and lessen the detrimental effects of an explosion.
In order to do that, a ventilation system has to continuously renew the air in the ventilated compartment, dilute any leaks of gas that may occur, and ensure that nuisance noise is kept to a minimum.
Moreover, leaks of gas in air extraction ducts may be due to potential leaks in the flanges of the pipework resulting from vibrations caused by the turbine or from thermal expansions due to excessive temperatures within the turbine.
When the compartment does exhibit potential leaks of gas, there are a number of devices that propose creating a reduced pressure in this compartment. Thus, the gases at high pressure are sucked out by the air at a lower pressure.
In order to comply with environmental standards, the ventilation systems have become increasingly complex, through the addition of several motor driven fan units, providing redundancy, the disadvantage here being the increase in the size and weight of such systems and making on-site assembly and maintenance more difficult.
The prior art has already attempted to achieve these objectives. Documents US 2004/0231418, JP 59015636 and US 2007/249279 in this regard propose solutions capable of achieving these objectives.
However, particular reference may be made to document US 2001/0003242 which proposes a ventilation system made up of fans and of ducts arranged vertically in the rear part of the turbine compartment.
This then reduces the size of the system and makes on-site assembly and maintenance easier.
Most conventional ventilation systems use non-return valves situated in the ducts, of round or square section. These valves are unable to seal the system completely and the frequent sticking of these valve shutters leads to air leaking between the fans, thus altering the flow and the profile of the ventilation. These shortcomings are due to the mechanical stresses applied on closing and opening these valves and to the vibrations caused by the passage of air through the valves in the air extraction duct.
Moreover, one gas turbine requires an air flow rate of 15 m3/s and an air flow axial to the turbine, whereas another device may require an air extraction flow rate of 2 m3/s with the air flow vertical to the device.
The solutions proposed in the prior art are unable to offer a ventilation system capable of adapting to suit different air extraction flow rates.